Fix strftime microseconds on Windows

[LunaMeerkats]

Summary

• replace the platform-dependent %Z placeholder with direct %f insertion
• support repeated %f directives while preserving escaped percent sequences
• extend scalar and array regression coverage

Root cause

Skyfield passed microsecond text through a synthetic tenth struct_time field
and formatted it as %Z. Windows ignores that supplied timezone value and
expands %Z using the host timezone, so %f rendered text such as
AUS Eastern Standard Time instead of six digits.

Fixes #1081.

Validation

• direct Windows 11 reproducer: 2025-05-14T05:15:36.894280Z
• focused microsecond regression, including repeated and escaped directives
• compatible full test run: 581 passed; the only 2 failures are pre-existing
  Windows failures where time.strftime() rejects year -746
• python -m pyflakes skyfield
• python -m compileall -q skyfield
• clean wheel build

The project's Assay runner imports the Unix-only fcntl module, so I used a
compatible local fixture-expansion runner on Windows. Linux validation remains
for project CI.

OpenAI Codex assisted with investigation, implementation, and validation. The
final diff and evidence were independently reviewed before publication.